The country of blank was made weaker by the Congress of Vienna's work

The correct answer to this open question is the following.

The country of France was made weaker by the Congress of Vienna's work.

At the end of the Napoleonic wars, European countries such as Great Britain, Austria, Prussia, and even Russia, met to rearrenge the continent after the war. They organized the Congress of Viena to reestablish some monarchies that Napoleon had overthrown. They worked from 1814 to 1815 and these nations could establish a relative peace that lasted until some years before World War I.